Coaxial cable typically includes an inner conductor, an outer conductor, a dielectric layer that separates the inner and outer conductors, and a jacket that surrounds the outer conductor. The outer conductor can take many forms, including flat, braided, and corrugated.
The dielectric layer is typically formed of a foamed polymeric material. However, in some instances attempts to increase the degree of air present in the dielectric layer have been made, as reduced material/increased air in the dielectric layer can reduce signal loss and/or increase the velocity of propagation of the signal. These approaches have some disadvantages, though, such as the inability to block the longitudinal migration of ingressed water, high material cost, poor reliability in the field, and slow manufacturing speed. For example, a prior coaxial cable 10 illustrated in FIG. 1 includes a continuous spiraling fin of solid polyethylene that serves as the dielectric layer 14 around a center conductor 12. An outer conductor 16 overlies the dielectric layer 14, and a polymeric jacket 18 overlies the outer conductor 16. In addition to being slow to manufacture, this design is incapable of blocking longitudinal migration of ingressed water. As another example, FIG. 2 illustrates a prior cable 10′ in which a center conductor 12′ is held at the center longitudinal axis by an array of surrounding PTFE tubes 14′ that serve as the dielectric layer, with an outer conductor 16′ and a jacket 18′ surrounding the tubes 14′. This design is also incapable of blocking ingressed water, and further has shown to be unreliable during heavy bending in the field; this unreliability is due to mechanical bending forces that cause the center conductor 12′ to displace an adjacent tube 14′, thus creating an electrical short between the inner and outer conductors. A similar design that employs “spokes” to maintain the center conductor in place also fails to block ingressed water. FIG. 3 illustrates a cable 10″ (discussed in U.S. Pat. No. 2,992,407 to Slusher, the disclosure of which is hereby incorporated by reference herein in its entirety) employing solid discs 14″ spaced along the center conductor 12″, with an outer conductor 16″ and a jacket 18″ completing the cable 10″. This design is capable of blocking water, but is relatively slow to manufacture, as the discs are individually injection-molded and positioned subsequently on the center conductor 12″. The solid discs 14″, typically formed of polyethylene, also contain significant mass to slow the velocity of propagation and degrade the electrical properties of the cable.
In view of the foregoing, it may be desirable to provide additional designs of dielectric layers utilizing air in the dielectric layer.